Conventional cross-point memory arrays include first and second sets of transverse electrodes with memory cells formed at the crossing-points of the first and second set of electrodes. Each of the memory cells includes, in at least one of its binary states, a diode. The diode is used as a current limiting device that prevents undesired flow of current through the memory cells, minimizing programming interference, programming disturbance, and read disturbances. Incorporation of a diode within the memory cells relaxes the constraints on the memory array, and improves performance, cost structure and achievable density.
However, conventional diodes have characteristics that are poorly suited for many applications. Conventional memory elements fabricated from, for example, phase change materials, require diodes capable of tolerating high current density. A diode with a high on/off ratio of less than 1e6 and capable of supplying a forward current of 100 A/cm2 is required in a conventional cross-point memory array. Additionally, conventional cross-point memory arrays include multiple stacked materials, which require formation using low temperature (i.e., less than 400° C.) processing. Therefore, the diode must be fabricated at temperatures of less than 400° C. or, alternatively, must be separately fabricated and interconnected with the cross-point memory array after formation. Moreover, the rigid substrates on which diodes are fabricated prohibits their use in applications in which the device must be physically deformed. Contaminants from metallic contact layers frequently react with the semiconductor body during processing, and degrade the diode's electrical characteristics. Consequently, fabricating a diode that meets the required specifications presents a challenge.
Electromechanical switches are suitable for integration into cross-point memory arrays as an alternative to diodes because of their excellent on/off ratios and fast switching characteristics. An electromechanical switch provides a physical separation between the switch and the capacitor making data leakage less severe. Due to limitations of conventional fabrication techniques, such as lithographic techniques, it is difficult to scale these devices. Thus, fabricating devices on a nanoscopic scale, often referred to as “nano-scale devices,” that function as ohmic contacts and have low resistance presents a challenge in semiconductor device fabrication. Conventional low resistance ohmic contacts are made of metal silicides formed on heavily doped semiconductor regions. The contact resistance is inversely proportional to contact area. In nano-scale devices, the contact area is on the order of one nanometer or smaller and, thus, contact resistance limits performance.
U.S. Published Application 2003/0122640 to Deligianni et al. describes a microelectromechanical switch having a movable part, two pairs of contacts, and actuators. The movable part is laterally or pivotally deflected by the actuators to make or break connections across pairs of contacts. Precise fabrication control is required to ensure that the actuator is movable within the required range without substantially deviating from the intended range and path of travel. The actuator experiences flexion stresses, which results in fatigue with long-term usage.
Dequesnes et al., titled “Simulation of Carbon Nanoelectromechanical Switches,” discloses a nanoelectromechanical switch that includes a single wall or a multiwall carbon nanotube and a fixed ground plane. Upon application of a voltage, electrostatic charges are induced on the carbon nanotube and the fixed ground plane that result in deflection of the carbon nanotube onto the ground plane. Dequesnes discloses that fixing both ends of the carbon nanotube decreases the significance of van der Waals forces between the carbon nanotube and the ground plane.
Jang et al., Applied Physics Letters, 87, 163114 (2005), discloses a nanoelectromechanical switching device including three multiwall carbon nanotubes (MWCNTs). Above a threshold bias, one of the MWCNTs makes contact with another of the MWCNTs, establishing an “on” state. Due to electrostatic forces and van der Waals forces between the MWCNTs, they are held together after the driving bias is removed.
In light of the state of the art, there is a need for nanoelectromechanical switching devices that may be formed at low temperatures, tolerate high current densities while providing reduced current leakage, and that eliminate the need for a negative bias, as well as methods that can be used to form such nanoelectromechanical switching devices.